Gain flattening filters are generally associated with optical amplifiers disposed in optical repeaters distributed regularly along transmission lines. Optical amplifiers do not usually apply equal amplification to all wavelengths of the signals transmitted on the various channels of the same transmission line. In particular, the development of dense wavelength division multiplexing (DWDM) transmission applications has tended to accentuate the amplification disparities over a given pass-band. It has therefore become necessary to produce gain flattening filters offering high contrast over narrow spectral bands to achieve perfect matching to the gain curve of in-line amplifiers, which are usually doped fiber amplifiers.
Gain flattening filters usually consist of Bragg gratings inscribed optically into optical fibers. An optical fiber conventionally comprises a core, whose function is to transmit and possibly to amplify an optical signal, surrounded by one or more cladding layers, whose function is to confine the optical signal in the core. To this end the refractive index n1 of the core and the refractive index n2 of the cladding are such that n1>n2. As is well-known to the person skilled in the art, the propagation of an optical signal in a fiber involves the propagation of a fundamental mode in the core and the propagation of secondary modes in the cladding.
To make it photosensitive, for inscribing a Bragg grating, the core and/or the cladding of the fiber can be doped, for example with germanium (Ge). The gratings conventionally used for gain flattening are slanted Bragg gratings (SBG). As shown in FIG. 1, in a slanted Bragg grating 5 the refractive index in the core 10 and/or the cladding 11 of a fiber portion 1 is modulated. The grating inscription angle θ is defined by the inclination of the optically inscribed index modulation to the propagation axis z of the optical signal and must be chosen to enable coupling of the fundamental mode into the cladding modes.
FIG. 2 shows the spectral response of a conventional SBG filter of the kind shown in FIG. 1. The parameters influencing the spectral response of an SBG filter include the inscription angle θ and the index step Δncore=n1−n2 between the core and the cladding of the fiber; θ=3.6° and Δncore=0.004 in the example shown.
DWDM applications require increasingly narrow filters offering increasingly accentuated contrasts.
In the case of an SBG filter, the best solution to reducing the spectral width of the filter whilst reducing reflection (caused by coupling of the fundamental mode to its contrapropagating self) is to improve the coupling between the fundamental mode and the cladding modes in order to increase significantly the integral of the overlap between the two modes. The overlap integral is defined as the area defined by the fundamental modes and the cladding modes weighted by the photosensitivity profile of the fiber.
Various prior art techniques have already been proposed for increasing the overlap area in a filter inscribed optically into a fiber portion.
A first prior art solution consists in increasing the diameter of the core or reducing the index step Δncore between the core and the cladding to widen the fundamental mode and thereby increase the overlap. However, this solution is limited by the loss of the monomode nature of the propagation of the signal if the diameter of the core becomes too large or by problems of coupling with the other optical components of the module.
A second prior art solution consists in making the cladding photosensitive as well as the core, to increase the weighting of the overlap area. However, this solution implies inscribing the Bragg grating with a large slant angle (at least 60) to prevent coupling of the fundamental mode to itself and thus total reflection of the signal.
For example, patent application WO 99/27401 describes an optical filter comprising a slanted grating inscribed in a fiber portion having a buried cladding, i.e. a cladding portion whose index is lower than that of silica, the cladding further being at least partly photosensitive, although less photosensitive than the core.
Another solution to reducing the spectral width of an SBG filter is to reduce the grating inscription angle. However, this solution is limited by the minimum angle required to guarantee non-reflection of the optical signal and to avoid the need for optical isolators, which are essential in the case of zero-back-reflection angle Bragg grating filters.
What is more, the problem of finding a filter having a reduced spectral width is compounded by the problem of the tolerance of the component to bending losses. Because of the trend for miniaturization of components, optical modules are increasingly compact and the fiber portions disposed in them are generally coiled or looped so that they occupy a small space. A filter inscribed optically in a fiber portion intended to be disposed in an optical module must therefore have some tolerance to bending losses.
A conventional solution to reducing bending losses in a fiber is to increase the index difference between the core and the cladding, that is to say to increase the index step Δncore=n1−n2.
However, a consequence of increasing the index step is to confine further the fundamental mode in the core, which then necessitates increasing the inscription angle in order to preserve a non-reflective filter. Increasing the inscription angle of the grating widens the spectral response of the filter. FIG. 3 illustrates this problem and represents the spectral response of an SBG filter inscribed in a fiber having an accentuated index step Δncore=0.0055 and an inscription angle θ=5°. Comparing FIGS. 2 and 3, it is seen that the spectral response of the filter has been widened, to the detriment of reducing bending losses.
One solution to reducing the inscription angle of an SBG whilst maintaining a high index step is described in the paper “Ultra narrow band optical fiber sidetap filter” by M. J. Holmes et al., ECOC'98. That kind of solution consists in making the cladding photosensitive and reducing the photosensitivity of the core, even to the point of the core having no photosensitivity.
This solution prevents coupling of the fundamental mode to itself in gratings with smaller angles. However, because the core is less photosensitive, the overlap and therefore the coupling of the fundamental mode with the cladding modes is greatly reduced. Also, the inscription angle to guarantee non-reflection depends on the difference in photosensitivity between the core and the cladding and the width of the mode, and is not easy to reproduce.